Search for a command to run...
This work forms part of a long-term study of the ecology and behaviour of the rook (Corvus frugilegus L.) in the Ythan Valley, Aberdeenshire. Its general aims were set out by Dunnet & Patterson (1968), who suggested that in north-east Scotland rook populations experienced their most serious food shortage during the summer. Lockie (1956), on the other hand, inferred from observations on food-fighting within flocks near Oxford that winter was critical. Patterson (1970) found that, within the Ythan Valley, fighting over food was most frequent during winter, particularly during snow when feeding sites became restricted, and this raised the problem whether fighting in rook flocks was a reliable indication of general food shortage, or whether it was a response to the high density of birds at localized feeding sites. Intense localization could result in some individuals getting insufficient food because they could not find space at the feeding sites, even though food might be superabundant there. As Lockie (1956a) pointed out, the establishment of a hierarchy through competition at feeding sites could ensure that the majority of the population obtained its daily food requirement, with only the most subordinate birds starving at any one time. In this situation, therefore, feeding space rather than food abundance would impose food shortage in some individuals. An alternative, with food being widely distributed but generally scarce, thereby imposing difficulties on all individuals within the population, appeared to be the situation (in the Ythan Valley) during the summer (Dunnet & Patterson 1968). These two kinds of food shortage would affect the individuals of a population in different ways and pose different problems for birds searching for food. Ward (1965) and Zahavi (1971) suggested that communal roosting was an adaptation to help individuals to discover localized feeding sites. As rooks roost communally it was decided to make observations which would distinguish between the two kinds of food shortage, and to see how the birds reacted to them, especially in terms of roosting behaviour and dispersal from roosts. The specific aims of this study were therefore: (i) to describe seasonal changes in the crop types available to the birds; (ii) to measure daily food intake at different times of year; (iii) to relate this to the daily food requirement of a captive rook in order to identify periods of food shortage in the field; and (iv) to observe variations in roosting behaviour and in dispersal from roosts. The study area consisted of about 10 sq. miles (c. 2500 ha) of arable land, comprising the common feeding areas of two rookeries, South Artrochie and Macharmuir, at Auchmacoy, near Newburgh, Aberdeenshire (Fig. 1). Spring sown barley and oats, winter